(A) About the Digital Cellular System
In the conventional digital cellular systems (PDC: Personal Digital Cellular telecommunication system, PHS: Personal Handy-phone System, GSM: Global System for Mobile communications, for example), the radio base stations and a plurality of mobile communication terminal stations carry out communications through time division multiplexed (TDMA) radio channels. In these systems, each radio station is assigned with a fixed frequency channel such that no interferences with neighboring base stations occur, and a plurality of mobile communication terminal stations and the radio base stations can carry out communications without causing interferences, by using channels formed by the frequency channels as a plurality of time division multiplexed (TDMA) channels.
In such a conventional radio communication system, in a case where a covered area by each base station has a multi-cell configuration, a radio communication for connecting a radio base station and a mobile communication terminal station within each cell, for example, is carried out though a channel frequency channel assigned to each base station. Then, in the radio communication in the FDMA or TDMA cellular scheme, in order to avoid the neighboring cell interferences, the frequency channel arrangement is made such that the identical frequency channel will not be used at the neighboring cells. Such conventional frequency channel assignment methods includes the following.
(1) Fixed Frequency Channel Assignment
In the fixed frequency channel assignment (FCA: Fixed Channel Assignment), the radio frequency channel that can be selected is determined fixedly for each radio cell, and it is configured such that the reuse of the radio frequency channel is realized with an optimal distance interval. The arrangement is made by determining the frequency channel to be arranged to each cell in advance, so that this is called the fixed frequency channel assignment method.
(2) Dynamic Frequency Channel Assignment
There is a method for dynamically rearranging the frequency channel to be assigned and arranged to each cell according to a traffic of each cell, with respect to said fixed frequency channel arrangement. This is called the dynamic frequency channel assignment. In this method, at each radio cell, all the frequency channels used by the system can be selected. Namely, as long as a required quality is satisfied, it can be used for communication at any radio cell.
In the dynamic frequency channel assignment method, there are advantages that
(a) the efficient utilization of frequencies according to coarse/dense of the traffic can be realized, and
(b) compared with the fixed frequency channel arrangement, the design is easier as there is no need to make the radio frequency channel arrangement plan before the start of the system operation.
(B) About the Frequency Orthogonal Multiplexing Scheme (OFDM)
On the other hand, in recent years, the multi-carrier transmission scheme is attracting attentions as a measure against the frequency selective fading in the high speed transmission. In this multi-carrier transmission scheme, the transmission data are transmitted by being distributed to a plurality of carriers with different frequencies, so that the band of each carrier becomes a narrow band, and it is harder to receive an influence of the frequency selective fading when the number of sub-carriers is larger. In particular the orthogonal frequency division multiplexing (OFDM: Orthogonal Frequency Division Multiplexing) in which respective sub-carriers are orthogonalized has a high frequency utilization efficiency and it is used for the radio LAN and digital broadcasting.
In this scheme, the frequencies of respective carriers are set such that respective carriers are mutually orthogonalized within a symbol section. The spectra of the OFDM signals are continuously overlapping with each other, and a processing to take out the signal on a particular carrier by using a band pass filter as in the ordinary multi-carrier transmission is not carried out. Then, in the OFDM, the orthogonalization of respective carriers and the extraction of each sub-carrier signal are carried out by using an inverse discrete Fourier transform (IDFT: Inverse Discrete Fourier Transform) circuit and a discrete Fourier transform (DFT: Discrete Fourier Transform) circuit in general.
Then, in the reception in this OFDM scheme, the data symbol is correctly taken out by the demodulation from each extracted sub-carrier, so that unlike the multi-carrier transmission in which a plurality of sub-carriers are arranged on a frequency axis by providing guard bands and each sub-carrier is separated by a narrow band filter, the frequency intervals of respective sub-frequency channels can be narrowed by overlapping them and therefore the frequency utilization efficiency is good in the OFDM.
The power spectrum of the general frequency division multiplexing scheme is, as shown in FIG. 1, formed by the arrangement of the occupied bands which are bands necessary for transmitting signals of respective sub-carriers and guard bands for preventing interferences between respective sub-carriers. In other words, the entire band used in the entire system is (Occupied band of the frequency channel)×N+(Guard band)×(N−1).
In the OFDM, the orthogonality between respective carriers is maintained and the overlap of respective modulated wave bands is made possible by setting the frequency interval of the sub-carrier to the interval of the first theorem of Nyquist. Namely, as shown in FIG. 2, the orthogonalized OFDM signal is such that the symbol can be taken out despite of the fact that the spectra of respective sub-carriers are overlapping, so that the frequency channel separation of the sub-carrier can be made narrow. In other words, the entire band used in the entire system is only (Occupied band of the frequency channel)×(N+1)/2.
In FIG. 3, a configuration of the conventional OFDM radio device using the inverse discrete Fourier transform circuit is shown. In FIG. 3, the data sequence to be transmitted is first applied with the baseband digital modulation by the symbol mapper 1. It is converted into a plurality of frequency channels of the identical symbol rate by the serial to parallel converter 2, and it is converted into a plurality of orthogonalized sub-carrier signals by carrying out the inverse Fourier transform by the inverse discrete Fourier transform circuit 4. The parallel output signals of the inverse Fourier transform circuit 4 are, the time series transmission signals are converted into the time series transmission signals by applying the serial to parallel conversion at the parallel to serial converter 5. They are converted into the RF frequency band used by the system at the radio transmitter 6, and after the power is amplified, they are transmitted through the transmission antenna 7.
Examples of the communication system in which the OFDM is introduced includes the radio LAN system using 5.2 GHz band, etc. In this system, 52 sets of the sub-carriers are used.
The OFDM has features such as the interferences between codes can be further reduced by setting the guard interval in the symbol section.
Also, usually, the sub-carrier is secured by the continuous band, In the OFDM transmission, the orthogonality between respective sub-carriers is very important, and if the orthogonality of frequencies is broken even slightly, the inter-carrier interference (ICI: Inter-Carrier Interference) occurs between sub-carriers, and it has a large influence on the signal transmission characteristics.
In the OFDM system used in the digital broadcasting, the orthogonality between respective sub-carriers is secured, and in the case of carrying out the plural station simultaneous transmission at the identical frequency channel, the sufficient synchronization is established between carries of the transmitter of each transmission site, and the sending of the broadcast signals is carried out such that the orthogonality can be secured sufficiently.
(C) About the Cellular System and the OFDM
As an example of applying the OFDM to the radio communication system, the radio communication system using the band division multiple access (BDMA: Band Division Multiple Access) scheme has been proposed, for example. The spectrum of the BDMA scheme is shown in FIG. 4. The BDMA scheme is a communication scheme which uses both the frequency division multiple access and the time division multiple access. In the BDMA scheme, the information transmission is carried out by applying the linear digital modulation such as QPSK to each sub-carrier. In this scheme, the entire transmission band is divided into a plurality of sub-bands, and they are assigned to different users in units of the divided sub-bands. Also, in Japanese Patent Application Laid Open No. H10-191431, it is proposed to increase or decrease the number of sub-carriers according to the transmission capacity required by the user in the multi-carrier transmission.
(D) About the CDMA Scheme
In the case of the CDMA, the arrangement of the identical frequency repeating cells is theoretically possible, but in the case where a plurality of micro-cells which communicate by using the identical frequency band exists within a macro-cell, the DSA (Dynamic Frequency arrangement: Dynamic Spectrum Allocation) is still necessary as a measure against the identical frequency channel interference within cell.
Now, in this CDMA scheme, there are cases of adopting the hierarchical cell structure such as that in which the micro-cells are located within the macro-cell, and in order to utilize frequencies efficiently in this hierarchical cell structure, there is a proposition for a method in which, in a system in which the identical frequency band is shared by systems with different transmission speeds for the micro-cell and the macro-cell, for example, when one of the frequency channels becomes unnecessary, a permission for use is given to one with the low priority level among the other one of the vacant frequency channel, and a partition which is a boundary between the frequency band of the micro-cell side and the frequency band of the macro-cell side is shifted (see Japanese Patent Application Laid Open No. H11-205848, for example).
However, in the conventional system such as the digital cellular scheme explained in the above described (A), there is a need to secure a plurality of frequency channels, and arrange them by providing a constant interval so as to avoid the interference of the identical frequency channels in the frequency channel assignment.
Consequently, it is utilized efficiently by assigning respective frequency channels to respective cells in the limited frequency band assigned to the system, but in the future the shortage of the number of frequency channels due to the increase of the data communication traffic is expected, and the radio communication system with the higher frequency utilization efficiency is demanded.
Also, in the OFDM explained in the above described (B), there is a need to secure the frequency channel band in which the sub-carriers satisfying the orthogonality condition are arranged continuously, as shown in FIG. 5. For this reason, the channel assignment is going to be regulated, and in the case of the shortage of the frequency channels, it is considered that it becomes difficult to deal with it flexibly.
On the other hand, in the digital broadcasting and the radio communication explained in the above described (C), a plurality of delayed waves with large time delays arrive due to the multi-path propagation, so that in the high speed transmission of information, the transmission speed required for the radio communication largely changes according to the user and the application, as from speech, electronic mails, still images, video image transfer, etc. Consequently, in the case of carrying out the information transmission by the bands that are divided and determined in advance as in the conventional radio communication system, the division loss becomes large, and there is a problem regarding the efficient utilization of frequencies.
In addition, in the CDMA scheme explained in the above described (D), even in the method for realizing the efficient utilization of frequency channels by shifting the partition, the total sum of the number of channels that can be secured in the system band is constant for the macro-cell and the micro-cell, and it is possible to consider the cases where the shortage of channels on one side cannot be compensated by channels of the other side by simply shifting the partition.
(Description of the Frequency Orthogonalized Multiplexing Scheme (OFDM))
Conventionally, the the multi-carrier transmission scheme has been proposed as a measure against the frequency selective fading in the high speed digital transmission. In this multi-carrier transmission scheme, the transmission information data is sent by being distributed to a plurality of sub-carriers with different frequencies, so that the band of each carrier can be a narrow band, and there is a characteristic that when the bands of these sub-carriers are narrower, it is harder to receive an influence of the waveform distortion due to the frequency selective fading.
In such a multi-carrier transmission scheme, particularly in the orthogonal frequency division multiplexing (OFDM: Orthogonal Frequency Division Multiplexing) in which respective sub-carriers are orthogonalized, unlike the conventional multi-carrier transmission in which a plurality of sub-carriers are arranged on the frequency axis by providing guard bands and each sub-carrier is separated by a narrow band filter, the frequency utilization efficiency can be made higher by narrowing the frequency intervals of respective sub-frequency channels by overlapping them, and it is used for the radio LAN and the digital broadcasting.
In this scheme, the frequency intervals of respective sub-carriers are set such that respective sub-carriers become orthogonal to each other within the symbol section. Then, in the OFDM, in practice, the orthogonalization of respective sub-carriers and the extraction of each sub-carrier signal are carried out by the digital signal processing, using the inverse discrete Fourier transform (IDFT: Inverse Discrete Fourier Transform) circuit and a discrete Fourier transform (DFT: Discrete Fourier Transform) circuit.
(Description of the OFDM Function Block)
FIG. 3 is a block diagram showing a structure of a conventional OFDM radio device equipped with an inverse discrete Fourier transform circuit.
As shown in FIG. 3, the conventional OFDM radio device has a symbol mapper 1 for applying the baseband digital modulation with respect to the information data sequence of the user, a serial to parallel converter 2 for converting the output signal of the symbol mapper 1 into a plurality of channels with the identical symbol rate, an inverse discrete Fourier transform circuit 4 for applying the inverse Fourier transform with respect to the parallel output signal sequences which are output signals of the serial to parallel converter 2 and converting them into a plurality of orthogonalized sub-carrier signals, a parallel to serial converter 5 for converting the output signals of the inverse discrete Fourier transform circuit 4 into a time series signal, and a radio transmitter 6 for converting it into an RF frequency band used by the system and amplifying the power.
In such a conventional OFDM radio device, assuming that the information data sequence to be transmitted is a, the digital modulation such as QPSK, QAM, for example is carried out by the symbol mapper 1 first. By this digital modulation, the information bit sequence is converted into a complex symbol sequence Sx (S0, S1, S2, S3). Next, it is distributed to a plurality (N sets) of sub-carrier channels (F1, F2, . . . FN) by the serial to parallel converter 2. Then, the inverse Fourier transform is carried out by the inverse discrete Fourier transform circuit 4, and they are converted into time series sample values (sample values of OFDM symbols) in which a plurality of orthogonalized sub-carrier frequency channel signals are superposed. The sample values of the OFDM symbols are serial to parallel converted by the parallel to serial converter 5, and converted into the continuous time series transmission signal, and after it is frequency converted into the RF frequency band used by the system at the radio transmitter 6, the power is amplified, and it is transmitted from the transmission antenna 7.
(Transmission Band Variable of the OFDM)
In the ordinary OFDM transmission device, the system clock frequency is fixed, so that the bandwidth is constant. For example, Japanese Patent Application Laid Open No. H11-215093 proposes a configuration for easily carrying out variable of the band frequency of the sub-carrier, and an addition of a function for automatically following variable of the band of the sub-carrier at a receiving side, in the transmission of the OFDM signals.
Describing it in detail, as shown in FIG. 6, in the OFDM transmission device disclosed in Japanese Patent Application Laid Open No. H11-215093, at the transmitting side, a clock output terminal of a clock oscillator 101B is connected to a clock rate conversion unit 101A, and a clock output terminal of the clock rate conversion unit 101A is connected to respective clock terminals of a rate conversion unit 101, an encoding unit 102T, an IFFT unit 103A, a guard attaching unit 103B, a synchronization symbol insertion unit 105, and an orthogonal modulation processing unit 108. At the receiving side, an output VC of a synchronization detector 109A is connected to a terminal VC of a clock oscillator 109B, and an output FSTr of the synchronization detector 109A is connected to FST terminals of an FFT unit 103C and a rate inverse conversion unit 107. Also, an output CKr of the clock oscillator 109B is connected to clock CK terminals of the FFT unit 103C, the rate inversion conversion unit 107, an orthogonal demodulation processing unit 109, and the synchronization detector 109A.
Then, in such an OFDM transmission device disclosed in Japanese Patent Application Laid Open No. H11-215093, the clock rate conversion unit for uniformly changing the operation timing of the transmission unit and the period of the clock that determines the clock rate is provided, and a function for controlling the reproduction clock rate according to the detected frame information period is added to the receiving side, and the frequency channel that can be used and the bandwidth that can be used are determined by checking the radio wave using state (how vacant the channels are).
Also, in Japanese Patent Application Laid Open No. 2000-303849, the flexibility and the adaptability of the OFDM system are given by making it possible to carry out the increasing/decreasing adjustment (scaling) for the operation parameters or characteristics of the system such as the symbol length of the OFDM, the number of carriers, or the number of bits per symbol of each carrier, for example, by the external setting or the decoded date.
The scaling control circuit of that OFDM system provides the compatibility or the desired performance by dynamically changing the operation parameters or characteristics according to the case of judging necessary or effective.
Also, in Japanese Patent Application Laid Open No. 2000-303849, the scaling of the OFDM parameter is carried out by the external setting or the decoded data. The scaling uses information such as a received signal strength, a ratio of the noise plus interference with respect to the received signal, a detected error, a notice, etc.
The influence of the multi-path in the case of adding the guard band is shown in FIG. 7. The advantage of providing the guard interval to the multiplexed carrier transmission is that it becomes possible to reduce or remove the inter-symbol interference (inter-code interference) doe to a signal dispersion (or a delay spread) in the transmission channel, as an interval as a guard time Tg is inserted while transmitting a next symbol and there is no need for a waveform equalizer that is necessary in the single carrier system.
The delayed copy for each symbol that arrives to a receiver after the intended signal can disappear before the next symbol is received, by the existence of the guard time. As such, the advantage of the OFDM lies in the function for overcoming the adverse influence of the multiplexed channel transmission without requiring the equalization.
In Japanese Patent Application Laid Open No. H11-215093 described above, the frequency that can be used and the bandwidth that can be used are determined by checking the radio wave using state (how vacant the channels are). However, in the actual mobile communications, the radio wave propagation path varies in time and its property changes largely. In the mobile communication propagation path, the following point becomes problematic by the delayed waves due to the time variation of the propagation path and the multi-path propagation, and there is a problem that the signal transmission characteristics are degraded.
By using FIG. 8, the relationship between the sub-carrier occupied bandwidth and the time variation of the fading will be described. Note that, the influence of the Doppler shift will be described as an example of the time variation of the fading.
In the mobile communication, the amount of the Doppler shift is determined by the moving speed of the mobile station itself, the moving speed of an object which reflected arriving radio waves, etc. In general, when the mobile station runs through the multiplexed wave propagation path, the received waves change randomly depending on the wavelength λ of the transmission waves and the moving speed V of the mobile station. Each element wave is Doppler shifted as much as V/λ=fD at maximum, and the spectrum spreading appears.
Here, assuming that the occupied frequency bandwidth of the narrow band sub-carrier (FIG. 8(a)) is B1, the occupied bandwidth of the wide band sub-carrier (FIG. 8(b)) is B2, and the maximum Doppler shift amount is DS, in the OFDM transmission in general, when the relative value of the Doppler shift amount DS with respect to the sub-carrier occupied bandwidth B becomes larger, the orthogonality between sub-carriers is deteriorated, and the signal transmission characteristics are degraded due to the inter-channel interference (ICI: Inter-Channel Interference). Namely, compared with the case where the occupied bandwidth of the sub-carrier is narrow (B1) as shown in FIG. 8(a), a rate (DS/BW) of the Doppler shift with respect to the sub-carrier band is sufficiently small in the case where the occupied bandwidth is wide (B2), so that the degradation of the transmission characteristics due to the Doppler shift is less.
However, when the band of the sub-carrier is wide, it becomes easier to receive the influence of the frequency selective fading due to the multi-path propagation. The frequency characteristics of the radio wave propagation path are affected largely by the propagation delay time, and when the maximum delay time τmax is large, the frequency characteristicd at the propagation path is distorted largely.
In contrast, as shown in FIG. 9, in the case where the occupied bandwidth of the sub-carrier is narrow as B1, it can be regarded as the uniform fading in each sub-carrier, and only the received signal levels of a part of the sub-carriers are lowered, so that it can be recovered to some extent by the gain adjustment by an AGC (Automatic Gain Control) circuit.
However, in the case where the occupied bandwidth of the sub-carrier is wide as B2, the received signal levels of a part of the occupied band drop frequency selectively due to the frequency selective fading, so that there has been a problem that the waveform distortion occurs and the signal transmission characteristics are considerably degraded.
Consequently, even in the case of carrying out the transmission with the identical information bit rate, the optimum sub-carrier bandwidth and the number of sub-carriers are different due to the time variation of the fading and the maximum delay amount. Also, in the case where the maximum tolerable bandwidth assigned to each user is constant, when each sub-carrier occupied band is widened, as shown in FIG. 10, there arise channels which deviate from the maximum tolerable bandwidth.
On the other hand, in the Japanese Patent Application Laid Open No. 2000-303849 described above, the scaling is carried out dynamically by the external setting or the decoded data. In other words, in order to avoid the frequency selective fading or the like in which the location factor is dominant, there has been a problem that there is a need to carry out the scaling by making some setting at each time of moving.
In addition, in the VSF-OFCDM scheme (Variable Spreading Factor-Orthogonal Frequency and Code Division Multiplexing) which is the so called fourth generation communication scheme, the information symbol is divided on a plurality of frequency axes, and the information symbol is transmitted by spreading it by the spread code of the variable spreading rate assigned to each mobile station, so that the symbol rate will be different from the other conventional transmission schemes so that the interferences between different transmission scheme occur in a region where the other transmission schemes coexist, and it is expected that the identical frequency band cannot be shared.